High molecular weight polyesters are commonly produced from low molecular weight polyesters of the same composition by solid state polymerization. The low molecular weight polyesters which are used in such solid state polymerizations are generally prepared by conventional melt polymerization. The molten polyester product of such melt polymerizations is convened to solid granules in the shape of pellets, chips, or cubes. Solid state polymerization of such pellets, chips, or cubes is generally considered advantageous in that the handling of high molecular weight ultra-high viscosity molten polymers is eliminated. Thermal degradation is also essentially avoided during the solid state portion of the polymerization.
Solid state polymerization of a polyester involves two major steps: chemical reaction and diffusion of reaction byproducts, e.g. water and ethylene glycol. Therefore, the solid state polymerization rate can be increased by reducing the diffusional resistance within the prepolymer granules. The diffusional resistance can be reduced by reducing the prepolymer particle size. However, smaller granules have higher tendencies to stick together during solid state polymerization, causing process difficulties. Therefore, there exists a minimum particle size that is suitable for solid state polymerization.
Several methods have been proposed to reduce diffusional resistance while maintaining a suitable particle size. U.S. Pat. No. 3,586,647 (Kremer) proposes foamed pellets which are formed by dispersing nitrogen or a foaming agent into the prepolymer melt before pelletizing. We have found that the improvement in the solid state polymerization rate of poly(ethylene terephthalate) (PET) by using foamed pellets is only between 20 and 35%. Because the cells within the foamed pellets are closed, the improvement in the solid state polymerization rate through use of foamed pellets will be somewhat limited. U.S. Pat. No. 4,755,587 (Rinehart) proposes porous pellets with interconnected voids. Porous PET pellets solid state polymerize 2 to 3 times as fast as standard solid PET pellets. Since the voids inside each porous pellet are interconnected, a great improvement in the solid state polymerization rate can be achieved by using porous pellets. Although porous pellets offer greatly improved solid state polymerization rates, the formation of porous pellets involves expensive operations-grinding, compacting, and classifying, etc.--and porous pellets tend to generate large amounts of fines, which impacts productivity.
Because of its high strength and barrier properties, poly(ethylene naphthalate) (PEN) is an excellent material for the applications of beverage and food containers and. industrial fibers. Due to its extremely high melt viscosity, PEN with an intrinsic viscosity suitable for such applications cannot be produced by melt state polymerization process alone. Instead, it must be produced by a combination of melt state and solid state polymerization processes. Because of the high barrier properties of PEN, the solid state polymerization rate is relatively slow. It is also known that PEN is preferably devolatilized prior to solid state polymerization, as described in U.S. Pat. No. 4,963,644 (Duh).
We have discovered that the solid state polymerization rate of PEN can be dramatically improved by using foamed PEN prepolymer granules. The magnitude of this rate improvement is surprisingly unexpected, even in view of the known improvement that results when foamed PET is solid state polymerized. In our investigation of the solid state polymerization rate of foamed PEN granules, we found that the solid state polymerization rate improvement by foamed PEN granules over standard solid PEN granules was surprisingly high--exceeding even the solid state polymerization rate improvement by porous PET granules over the standard solid PET granules, if the foamed PEN granules are devolatilized prior to crystallization and solid state polymerization. The use of a foamed PEN prepolymer, combined with a devolatilization step prior to solid state polymerization, provides a particularly fast and productive solid state polymerization process for a PEN polymer.